The most toxic dioxin is called 2,3,7,8-TCDD. Dioxins are produced as by products of the manufacture of some herbicides
(for example, 2,4,5-T)
How can we judge the toxicity of dioxin (or of any chemical, for
that matter)? One way is to look at the standards that have been set by regulatory agencies.
In the case of dioxin, EPA has calculated a "safe" dose, taking
into consideration dioxin's ability to cause cancer. The "safe" dose is expressed in extremely small units: femtograms. There
are 28 grams in an ounce, and one femtogram is 0.000,000,000,000,001 grams, or one quadrillionth of a gram, or 10**-15 (or,
10 raised to the power of negative 15) grams.
EPA believes that ingesting (eating) 6.4 femtograms (6.4 x 10**-15
grams) of 2,3,7,8-TCDD per kilogram of body weight per day would cause cancer in one in a million people so exposed (pg. 95).
Since an average adult weighs 62 kilograms or 137 pounds (average men weigh 70 kilograms [154 pounds] and average women weigh
55 kg [120 pounds]), the EPA is saying that 397 femtograms of 2,3,7,8-TCDD consumed in food each day would kill one-in-a-million
humans so exposed. Over a year's time, 397 femtograms per day add up to 145,000 femtograms; over a 70-year lifetime, this
would add up to 10.1 million femtograms, so 10.1 million femtograms (or 0.01 micrograms) is the maximum amount you could safely
get into your body during your entire lifetime, EPA believes.
can we express this in terms that people can grasp?
Let's compare it to one single aspirin tablet. One aspirin tablet
weighs 5 grains (or 325 milligrams, or 325 trillion femtograms), so to express one "safe" lifetime dose of 2,3,7,8-TCDD, you
would take a single aspirin tablet and divide it into 32 million (actually 32,172,218) miniscule pieces. Then one of those
tiny pieces would represent one "safe" lifetime dose of 2,3,7,8-TCDD.
Another comparison: A single grain of table salt weighs approximately
0.1 milligrams or 100 billion femtograms, so to get an amount of table salt that weighs the same amount as one "safe" lifetime
dose of 2,3,7,8-TCDD, you would divide a single grain of table salt into 9,900 microscopic pieces. One of those tiny pieces
would represent a "safe" lifetime dose of dioxin.
The U.S. Food and Drug Administration (FDA) has its own way of calculating
the same one-in-a-million cancer risk and they believe the EPA has overestimated the hazard by a factor of 10. In other words,
FDA believes you could represent a "safe" dose of 2,3,7,8-TCDD by dividing a single grain of table salt into 990 pieces, with
one of those pieces representing a safe lifetime dose. The federal Centers for Disease Control (CDC) in Atlanta has done its
own calculation, concluding that the cancer hazard from dioxin is about half-way between the EPA's estimate and the FDA's
estimate. EPA says 6.4 femtograms per kilogram of body weight per day is the safe dose; CDC says the correct number is 27.6;
FDA says it's 57.2 (pg. 95). No matter which agency does the calculation, there's no escaping the fact that dioxin is considered
One other way to understand the toxicity of dioxin is to compare
the dioxin "reference dose" established by EPA to the "reference dose" they have set for other common toxic materials. The
"reference dose" is the highest amount they believe you could eat regularly without incurring any disease (not considering
The reference dose for dioxin is 0.000,000,001 milligrams per kilogram
of body weight per day (mg/kg/day) (pg. 94); the reference dose for the toxic metal cadmium is 0.001 mg/kg/day and the "reference
dose" for the toxic metal arsenic is the same as for cadmium. Thus we can see that EPA considers dioxin in food 1,000,000
times (one million times) more toxic than cadmium or arsenic, not counting the cancer hazard from dioxin. Yes, dioxin is toxic, no doubt about it.
--Peter Montague, Ph.D.